This is too difficult of a question to answer since A) I don't know what you look like and B) what I find attractive in a girl may be unattractive to another guy.
You can't generalize attraction.
It's like me asking: Hey I am kind of funny how do I become funnier! When there are different people from different cultures with different sense of humor, like feminist humor may make one giggle and another frown.
Get my point?
But if ya want a general answer to your general question you become more attractive with a smile.
Smile. Smiling always makes somebody more attractive!
Exercise regularly. The healthy look that somebody gets after active exercise, the smile in the eyes, the freshness of movement, is attractive. You don't have to be super fit, but staying active is good.
Be busy, with a little bit of work and a good portion of social fun with friends. People like people who aren't needy and who have healthy relationships around them.
Work out! Eat healthy and be healthy. Wear clothes that looks nice and make your body look nice. Clothes can basically make you cool even if you are not lol. Have a haircut that suits your face. NEVER put too much make-up 9/10 guys find that gross and will think you are slutty. Basically just enhance your natural you. Be moderate about it too :) But being healthy, having a good hygiene and dressing well goes a very long way :)